Influence of apparent molecular size from aquatic humic substances in the coagulation efficiency with aluminium sulfate and ferric chloride / Influência do tamanho molecular aparente das substâncias húmicas aquáticas na eficiência da coagulação com sulfato de alumínio e cloreto férrico

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Water sources used to supply the public water system frequently have a relatively high true color intensity due to dissolved aquatic humic substances (AHS). In this study, water samples were collected from the Itapanhaú River (Bertioga, SP, Brazil), which exhibited a true color intensity in the order of 300 Hanzen units. XAD-8 resin was used to extract AHS. The AHS extract was filtered through a membrane with 0.45 µm pores, giving fraction F1, and this was separated by ultrafiltration into 4 apparent molecular size fractions of humic material: F2 from 100 kDa to 0.45 µm, F3 from 30 to 100 kDa, F4 from 10 to 30 kDa and F5 from 5 to 10 kDa. The fractions were characterized by elemental analysis and UV/Vis, infra-red (IR) and carbon 13 nuclear magnetic resonance (13C-NMR) spectroscopy. In general, the results showed that the smaller molecular size fractions (F3, F4 and F5) had a higher proportion of aliphatic than aromatic carbon atoms and a relatively high percentage of oxygen atoms bonded to alkyl groups and in carboxylic acids. Conversely, the apparently larger molecules (fractions F1 and F2) showed a higher content of aromatic groups. In the case of the 13C-NMR spectra, no significant differences could be detected among the fractions. There were stronger peaks in the carboxyl group and aliphatic carbon region and weaker peaks in the aromatic region, indicating that in the AHS as a whole, there is a greater content of oxygenated groups and aliphatic carbon atoms. Water from an artesian well was used to prepare experimental samples of the following molecular size fractions of AHS: F1 <0.45µm, F2 from 100 kDa to 0.45 µm and F3: from 30 kDa to 100 kDa and F4: <30 kDa, with true color intensity around 100 Hazen units and turbidity around 5.0 NTU. To assess the influence of the apparent molecular size on the efficiency of coagulation, jar test were carried out, using direct filtration. The coagulants employed were aluminum sulfate and ferric chloride. The coagulation diagrams obtained with these products revealed the effects of the molecular size of the AHS on the coagulation process. For the experimental samples, fractions F3 e F4 , a greater dose of coagulant was needed to remove the apparent water color around 5.0 Hanzen units and, even then, the amount removed was smaller. F4also had a higher proportion of fulvic acids, which exhibited a larger number of negatively-charged groups. For these reasons, a high dose of coagulant is necessary to achieve an efficient removal of apparent color.

ASSUNTO(S)

remoção de cor color removal coagulation substâncias húmicas aquáticas aquatic humic substances coagulação caracterização characterization

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